This invention relates generally to the drying of pulverized materials and is particularly directed to the drying of a pulverized material by means of a heated condensable vapor at relatively low pressure in combination with a vapor sink to separate and remove the moisture from the pulverized material.
Drying mills adapted to receive wet pulverized material in particulate form, such as in a slurry or the like, are commonly used to dry a wide range of materials such as grains, seeds, sand and coal. The wet particles are fed into one end of an inlet chamber, preferably in the form of an atomized spray, and are entrained in a typically hot gaseous fluid. As the particles are entrained in the hot angularly directed gases, they are centrifugally whirled through a generally arcuate path leading from the inlet chamber. During their travel with the hot gases in this generally arcuate path, a large portion of the particles are completely dried while some are only partially dried. The lighter, completely dried particles are displaced around an inner portion of the centrifugal path, while the heavier, wetter particles whirl around an outer portion of the centrifugal path. Such systems frequently include a classification section wherein, as the particles transit the apparatus, the lighter particles in the inner centrifugal portion pass through an exhaust port while the heavier particles continue down the inlet chamber where they are mixed with newly introduced pulverized material and are again entrained by the hot gaseous fluid and recycled through the drying mill.
The hot treatment gas is typically introduced tangentially into a cylindrical chamber and travels in a helical path along the longitudinal direction thereof. The raw, moist material is fed into the gas flow and is thereby subjected to a centrifugal force field within the chamber. The dust-like product thus subjected to the centrifugal force field advances through the hot treatment gas in a spiraling path following the gas flow and moving transversely, or in a cross-current, to the hot treatment gas from the interior of the cylindrical chamber toward the outside thereof to the chamber wall. The chamber wall is generally provided with an opening at one end thereof through which the preheated or chemically treated material is discharged from the cylindrical chamber. The transverse flow direction, or cross-current guidance, of the heated material and gas mixture results in higher heat exchanges than available with a simple direct flow arrangement. The kinetic and thermal energy present at the entrance end of the cylindrical chamber thus serves essentially to accelerate, disperse, and preheat the particles of material, whereas the gas flow at the exit end of the cylindrical chamber functions primarily to raise the temperature of the pulverized material in removing moisture therefrom. This type of drying apparatus utilizes the so-called tornado-flow principle and is generally termed a rotationally symmetrical vortex chamber.
Attempts to use the cyclonic flow devices described above in the processing of coal fines have met with limited success. In a coal drying operation, the particles are so small that they become entrained within the gas flow and escape the rotationally symmetrical vortex chamber at the gas outlet and require a subsequent cyclonic flow stage or two for separation from the effluent gas. Separation could also be accomplished by means of a water scrubber spray. However, this approach would put the coal particles in the same form as provided to the rotationally symmetrical vortex chamber, i.e., in a wet slurry or partially dried agglomeration. In fact, the coal dust is typically originally recovered from the combustion exhaust gases by means of a water scrubber spray. Thus, the use of a conventional rotationally symmetrical vortex chamber would result in an endless cycle and would be of little practical use in the recovery processing of coal fines.
Following combustion gas heating of the coal, the fine coal dust is recovered from the combustion exhaust gases typically by means of a water scrubber spray as described above. This process produces a wet slurry typically comprised of approximately 80% water. The slurry is then allowed to dry primarily by evaporation which reduces its moisture content to approximately 40%. However, in order to be commercially usable, the moisture content within the coal fines must be less than 10%. To date, there is no efficient means or method available for reducing the moisture content of coal fines down to commercially usable levels.
Therefore, the present invention is intended to provide an efficient and effective means and method for removing substantially all of the moisture from coal fines in producing commercially usable coal. While disclosed primarily as related to the drying of coal fines, the present invention is not limited to such applications and may be used equally as well for drying substantially any pulverized material efficiently and at low cost.